The present invention relates to reclosable packaging and in particular to such packaging having an interior volume that is impermeable to gas, thereby protecting perishable contents from spoilage. Typically, the package has a slider-operated zipper or has pull flanges above the interlocking elements of the zipper.
In the use of plastic bags and packages, particularly for foodstuffs, it is important that the bag be hermetically sealed until the purchaser acquires the bag and its contents, takes them home, and opens the bag or package for the first time. It is then commercially attractive and useful for the consumer that the bag or package be reclosable so that its contents may be protected. Flexible plastic zippers have proven to be excellent for reclosable bags, because they may be manufactured with high-speed equipment and are reliable for repeated reuse.
A typical zipper is one that has a groove at one side of the bag mouth and a rib at the other side, which rib interlocks into the groove when the sides of the mouth of the bag are pressed together. Alternatively, a member having a plurality of ribs may be on one side of the bag mouth, while a member having a plurality of channels may be on the other side, the ribs locking into the respective channels when the sides of the mouth of the bag are pressed together. In such a case, there may be no difference in appearance between the two members, as the ribs may simply be the intervals between channels on a strip that may lock into another of the same kind. In general, and in short, some form of male/female interengagement is used to join the two sides of the bag mouth together. The so-called members, or strips, are bonded in some manner to the material from which the bags themselves are manufactured. Usually, pull flanges extend above the rib and groove strips, which pull flanges may be pulled apart for access to the interior of the bag.
Although flexible zippers of this variety are quite popular, they do not always prevent the admission of ambient air into the interior volume of the package. In particular, ambient air can enter the interior volume of the package via the interstices of the interlocked profiled fastener strips or zipper halves (hereinafter xe2x80x9cprofiled interlocking membersxe2x80x9d), which usually do not form a hermetic seal. In the event that a tamper-evident easy-open gas-permeable membrane is incorporated below the zipper, this also will not hermetically seal the interior volume of the package. The presence of gases such as oxygen, nitrogen and carbon dioxide inside the package is undesirable, especially when the contents of the package are perishable. It is known in the prior art to provide a zipper flange comprising gas-impermeable material for hermetically sealing the package. There is a need for a hermetically sealed zippered package that can be readily torn open by the consumer. Such zippered package designs should also allow the package to be formed on conventional packaging equipment with little or no modification of the equipment being required.
The present invention is directed to a flexible zipper comprising gas-impermeable thermoplastic material, preferably incorporated in one or both flanges of the zipper. The invention is further directed to a reclosable package comprising gas-impermeable web material shaped to form a receptacle and a zipper comprising a layer of gas-impermeable material. The gas-impermeable web material of the receptacle and the layer of gas-impermeable material of the zipper form a substantially gastight enclosure for preserving perishable contents packaged therein.
In accordance with one embodiment, a flexible zipper comprises first and second profiled interlocking members and first and second flanges respectively joined to the first and second profiled interlocking members, wherein one flange or both flanges comprise a material that is gas impermeable. Preferably, one or both flanges has a layered structure comprising a layer made of a gas-permeable material and a layer made of said gas-impermeable material. In one embodiment in which only one flange has this layered structure, that flange includes a thinned area where the layer of gas-permeable material is absent. In another embodiment in which both flanges have the aforementioned layered structure, a gap between opposing edges of the gas-impermeable layers of the flanges is spanned by a membrane made of the same gas-impermeable material, the layers and membrane of gas-impermeable material forming a continuous gas barrier. The membrane has a thickness less than the thickness of the layered structures and thin enough to facilitate easy tearing by a consumer when the zipper is incorporated in a package. In accordance with the embodiments, this thinned tear zone made of gas-impermeable material extends in a longitudinal direction of the zipper.
In accordance with further embodiments of the invention, a reclosable package comprises a gas-impermeable web material defining a receptacle having a mouth at an upper end and a flexible zipper attached to the web material at the mouth, the zipper having one of the structures described above. The zipper comprises a continuous layer of gas-impermeable material that, when the zipper is installed in the package, serves as a gas barrier to protect perishable contents inside the package. In accordance with one embodiment, a single zipper flange acts as the gas barrier. In accordance with another embodiment, two flanges in combination with a connecting membrane act as the gas barrier.
In accordance with another aspect of the invention, methods of manufacturing the zippers disclosed herein involve coextrusion of gas-permeable and gas-impermeable thermoplastic materials. Alternatively, a zipper with gas-permeable and gas-impermeable layers can be formed by lamination. The preferred gas-permeable thermoplastics are polyethylene and polypropylene; the preferred gas-impermeable thermoplastics are nylon, polyester, polyvinyl dichloride and ethylene vinyl alcohol.
In cases of coextrusion of gas-permeable and gas-impermeable thermoplastic materials, the thinned area in a zipper flange (or the membrane connecting two zipper flanges) can be formed by shaping the extrusion die. The die orifice may be formed with a constriction where a thinned area is to be formed. The resulting thinned area will have a gas-impermeable layer and a gas-permeable layer. Optionally, gas-permeable layers can be coextruded on both sides of the gas-impermeable layer.
Other aspects of the invention are disclosed and claimed below.